Our founding fathers were Deists. That's right, I said Deists. Not Christians. This is an unsettlingly common misconception about the founding fathers, and I constantly hear the argument that we were founded by Christians.
The majority of U.S. citizens are currently Christian, but that does not mean we should identify as a Christian nation. It would completely undermine the U.S. constitution.
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or of the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
-United States Constitution
Identifying as a Christian nation would alienate non Christians (such as myself) from the United States. Last time I checked, we are a nation united as AMERICANS, not Christians.
Let's look at the original pledge of Allegiance.
"I pledge allegiance to the Flag of the United States of America, and to the Republic for which it stands, one Nation under God, indivisible, with liberty and justice for all."
How can we call ourselves indivisible if we identify as a Christian nation?
Although politicians like to call the United States a "Christian nation", that isn't actually the case. Christian men formed this nation, but the U.S. is designed to operate free from religious interference. Plenty of people are religious, but hundreds of religions exist in the country. Therefore, nobody can correctly call the U.S. a Christian nation.